1. Technical Field
The present invention relates to a medium transportation device that transports media such as sheets of paper for printing and a recording apparatus.
2. Related Art
JP-A-2007-217176 and JP-A-2003-267591 disclose a recording apparatus including a transportation device that transports media (recording media) such as sheets of paper and a recording unit such as a recording head that performs printing on a portion of the medium which is held by a medium holding member while the medium is transported by the transportation device.
In the recording apparatus (image forming device) described in JP-A-2007-217176, the images of the surface feature of the transported medium are sequentially captured by using a camera (imaging device), and two images in a time series are compared so as to calculate the transportation distance of a target pattern in the image. Then, the actual transportation distance of the medium is calculated by summing the transportation distances of the target pattern.
Further, in the recording apparatus (printer) described in JP-A-2003-267591, laser light is emitted onto the medium such as a paper sheet. The transportation distance of the medium is detected by comparing speckle patterns generated in the light reflected from the medium in a time series.
In the medium transportation device of the recording apparatus described in JP-A-2007-217176, the camera is disposed on the side of a recording surface of the medium at a position downstream with respect to a pair of transportation rollers in the transportation direction. If the medium is lifted from a holding surface of the medium holding member, for example, the distance between the camera and the medium changes and the image captured by the camera becomes out of focus. As a result, the image of the medium becomes blurred, which causes a problem that the detection accuracy of the transportation distance of the medium decreases.
Further, the medium transportation device of the recording apparatus described in JP-A-2003-267591 has a similar problem in that, if the medium is lifted from a holding surface of the medium holding member, the speckle pattern generated in the light reflected from the medium is displaced and the detection accuracy of the transportation distance of the medium decreases.
Particularly, when the leading edge of the medium does not reach a downstream pair of transportation rollers, which is a component of the transportation unit, immediately after the medium is fed out in a transportation process, the medium is often lifted from the holding surface, since the medium is held between an upstream pair of transportation rollers only. Further, in JP-A-2007-217176 and JP-A-2003-267591, a portion of the medium which is not supported on the medium holding surface serves as a detection area. This causes a problem that the detection accuracy of the medium transportation distance tends to decrease, since the detection area of the medium may be displaced not only in the direction away from the holding surface but also in the opposite direction and the paper sheet may be easily out of the expected transportation path.